linux-kernel - Re: [PATCH v3] xen/balloon: don't online new memory initially

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Date:   Tue, 18 Jul 2017 12:08:25 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v3] xen/balloon: don't online new memory initially

On 07/10/2017 04:10 AM, Juergen Gross wrote:
> When setting up the Xenstore watch for the memory target size the new
> watch will fire at once. Don't try to reach the configured target size
> by onlining new memory in this case, as the current memory size will
> be smaller in almost all cases due to e.g. BIOS reserved pages.
>
> Onlining new memory will lead to more problems e.g. undesired conflicts
> with NVMe devices meant to be operated as block devices.
>
> Instead remember the difference between target size and current size
> when the watch fires for the first time and apply it to any further
> size changes, too.

I don't think I understand how deferring setting the target until next
watch firing will help with the original problem. Could you explain?

Thanks.
-boris


>
> In order to avoid races between balloon.c and xen-balloon.c init calls
> do the xen-balloon.c initialization from balloon.c.
>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  drivers/xen/balloon.c     |  3 +++
>  drivers/xen/xen-balloon.c | 22 ++++++++++++----------
>  include/xen/balloon.h     |  8 ++++++++
>  3 files changed, 23 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index 50dcb68d8070..ab609255a0f3 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -780,6 +780,9 @@ static int __init balloon_init(void)
>  	}
>  #endif
>  
> +	/* Init the xen-balloon driver. */
> +	xen_balloon_init();
> +
>  	return 0;
>  }
>  subsys_initcall(balloon_init);
> diff --git a/drivers/xen/xen-balloon.c b/drivers/xen/xen-balloon.c
> index e7715cb62eef..e89136ab851e 100644
> --- a/drivers/xen/xen-balloon.c
> +++ b/drivers/xen/xen-balloon.c
> @@ -59,6 +59,8 @@ static void watch_target(struct xenbus_watch *watch,
>  {
>  	unsigned long long new_target;
>  	int err;
> +	static bool watch_fired;
> +	static long target_diff;
>  
>  	err = xenbus_scanf(XBT_NIL, "memory", "target", "%llu", &new_target);
>  	if (err != 1) {
> @@ -69,7 +71,14 @@ static void watch_target(struct xenbus_watch *watch,
>  	/* The given memory/target value is in KiB, so it needs converting to
>  	 * pages. PAGE_SHIFT converts bytes to pages, hence PAGE_SHIFT - 10.
>  	 */
> -	balloon_set_new_target(new_target >> (PAGE_SHIFT - 10));
> +	new_target >>= PAGE_SHIFT - 10;
> +	if (watch_fired) {
> +		balloon_set_new_target(new_target - target_diff);
> +		return;
> +	}
> +
> +	watch_fired = true;
> +	target_diff = new_target - balloon_stats.target_pages;
>  }
>  static struct xenbus_watch target_watch = {
>  	.node = "memory/target",
> @@ -94,22 +103,15 @@ static struct notifier_block xenstore_notifier = {
>  	.notifier_call = balloon_init_watcher,
>  };
>  
> -static int __init balloon_init(void)
> +void xen_balloon_init(void)
>  {
> -	if (!xen_domain())
> -		return -ENODEV;
> -
> -	pr_info("Initialising balloon driver\n");
> -
>  	register_balloon(&balloon_dev);
>  
>  	register_xen_selfballooning(&balloon_dev);
>  
>  	register_xenstore_notifier(&xenstore_notifier);
> -
> -	return 0;
>  }
> -subsys_initcall(balloon_init);
> +EXPORT_SYMBOL_GPL(xen_balloon_init);
>  
>  #define BALLOON_SHOW(name, format, args...)				\
>  	static ssize_t show_##name(struct device *dev,			\
> diff --git a/include/xen/balloon.h b/include/xen/balloon.h
> index d1767dfb0d95..8906361bb50c 100644
> --- a/include/xen/balloon.h
> +++ b/include/xen/balloon.h
> @@ -35,3 +35,11 @@ static inline int register_xen_selfballooning(struct device *dev)
>  	return -ENOSYS;
>  }
>  #endif
> +
> +#ifdef CONFIG_XEN_BALLOON
> +void xen_balloon_init(void);
> +#else
> +static inline void xen_balloon_init(void)
> +{
> +}
> +#endif